The present invention relates to a method of making fibrous media of controlled porosity, and more particularly such a media which combines high vapor permeability and low liquid permeability, and the product thereof.
It is well known to produce a laminate made from various polymers and textiles for use in a wide variety of product applications. For example, meltblown and spunbond materials afford a high level of vapor permeability and liquid permeability when used either by themselves or in combination with one another and/or other porous materials.
Meltblowing is a method for economically producing very small fibers which are mostly suitable for filtration and insulation applications. Fibers smaller than 1 micron in diameter may be produced by meltblowing, and the average fiber diameter in conventional meltblowing is about 4 microns, with fiber size distribution ranging from xc2xcmicron to 8 microns. To form such small fibers one must start with polymer resins of very low molecular weight. In this process the nonwoven fabric is formed in one step from the polymer resin into the final meltblown nonwoven fabric.
In contrast, spunbonding is very similar to conventional fiber spinning where several processing steps are required to form the spunbond fabric. Spunbond fibers go through a drawing stage and then a laydown stage wherein the drawn fibers are laid down into a matt and the matt is then bonded by a thermobonding calender or mechanical entangling to form the nonwoven fabric. The resins used in the spunbonding process have lower molecular weight than those used in the conventional melt spinning process and higher molecular weights than those used in the conventional meltblowing process. Fibers smaller than 10 microns in diameter are very difficult to produce economically by spunbonding, and the average fiber diameter for conventional spunbonding processes is about 18 microns.
However, for particular applications, such as those in the health care industryxe2x80x94e.g., infant diapers, sanitary pads, adult incontinence wear, medical surgical dressings, and the likexe2x80x94the laminate must perform three distinct functions: First, a topsheet intended to contact the patient""s skin must allow the passage of moisture (e.g., blood, urine and like liquids) therethrough while at the same time providing an acceptable feel to the wearer""s skin. Second, an absorbent core, intermediate the topsheet and the backsheet, must be capable of absorbing the moisture which has been received through the frontsheet. Third, a backsheet, on the back of the absorbent core, prevents leakage of moisture outwardly of the laminate. The present invention relates specifically to the backsheet component.
The barrier properties of the backsheet (i.e., the trapping of moisture and other liquids) are typically achieved by incorporating into the backsheet a plastic layer or film which acts as a moisture barrier. Various major disadvantages associated with the utilization of such barrier films are the low moisture vapor transmission rates (MVTR) of the barrier films, undesirable crinkling noise created by the barrier film during usage of the product, and a stiffening of the product (due to the barrier film) which reduces its conformability to the area to which it is applied.
Porous films are typically permeable to both liquid water and water vapor. They may be made by the incorporation of different organic or inorganic additives into a polymer film, the film then being stretched or fillers removed therefrom chemically. Other conventional methods include mechanical perforation and/or radiation techniques to form the desired holes or slits in the polymeric film. Formation of uniform pore size in a film is very difficult, and porous plastic films are generally more expensive than non-wovens.
On the other hand, non-porous barrier films are typically impermeable to both liquid water and water vapor. As a result, using the impermeable film in a diaper backsheet, for example, makes the diaper hot before exposure to liquid (as the barrier film prevents air circulation) and clammy after exposure to moisture (because the barrier film precludes moisture evaporation). Indeed, the use of an impermeable barrier film in a diaper may cause severe dermatological problems, such as skin rash on infants, and skin sores on adults wearing such non-porous products.
It is also known to form a semi-porous barrier film of controlled porosity which is permeable to water vapor, but impermeable to liquid waterxe2x80x94that is, breathable. However, the method of manufacturing such a microporous film of controlled porosity is typically complex and expensive, and requires a relatively specialized polymeric input (for example, conjugate fibers formed of two separately manufactured polymeric materials or laminates formed of two separately manufactured polymeric materials).
Clearly the need remains for a method of economically manufacturing a media of controlled porosity, combining high vapor permeability and low liquid permeability, without the use of chemical binders, additives or coatings, from a single commercially available polymer. Such breathable media would find use in products which are sold in such quantity that any reduction in the cost thereof (e.g., which makes it sufficiently economical for manufacture for use in disposable products) is highly desirable.
Accordingly, it is an object of the present invention to provide a method of making of a media of controlled porosity, combining high vapor permeability and low liquid permeability.
Another object is to provide such a method which does not require a specialized polymeric input.
A further object is to provide such a method which does not require the use of chemical binders, additives or coatings to provide the desired permeability or porosity.
It is also an object of the present invention to provide a material made by the aforesaid method.
It is another object to provide such a material which does not produce noise during use and which exhibits cloth-like feel (hand).
It is a further object to provide such a material which is economical to manufacture (e.g., for use in disposable products).
It has now been found that the above and related objects of the present invention are obtained in a method of making a non-woven fibrous media combining high vapor permeability and low liquid permeability. The method comprises the steps of providing a non-woven fabric formed from fibers that are prematurely crystalized during web formation and have a wide heat of fusion range distribution, and then calendering the fabric to soften the small polymer crystals therein of low heats of fusion, but not the relatively larger crystals therein of relatively higher heats of fusion, thereby to retain high vapor permeability.
The present invention additionally comprises a non-woven fibrous media providing high vapor permeability and low liquid permeability. The material is a non-woven fabric formed from fibers that are prematurely crystalized polymer and have a wide heat of fusion range distribution. The fabric is calendered to soften the small polymer crystals therein of low heats of fusion, but not the relatively large polymer crystals therein of relatively higher heats of fusion, thereby to retain high vapor permeability while providing low liquid permeability.
In a preferred embodiment the polymer is polypropylene, and optimally isotactic polypropylene, although other isotactic polymers may be used. The polymer is prematurely crystalized, preferably by quenching it prior to completion of fiber structural formation so that the polymer exhibits a bell-shaped heat of fusion range distribution (prior to calendering). The temperature, pressure and roller speed of the calendering operation are selected to soften the small polymer crystals, but not the relatively larger polymer crystals. For example, the fabric is preferably calendered at a temperature of about 25-110xc2x0 C., a pressure of about 25-150 Newtons/mm, and a roller speed of up to 200 meters/minute. The fabric is calendered to retain a moisture vapor permeability of at least about 1200 g/m2 @ 24 hours and to provide a hydrostatic head of at least about 10 millibars (about 100 mm H2O). The calendered material may be made into a composite with, for example, at least one spunbond, spunmelt or other nonwoven fabric layer.